Electrospray is a known process by which small charged droplets are formed from liquid ejected from a capillary orifice, or jet. By subjecting the liquid emerging from the jet to a strong electric field, the ejected particles become charged. If the charge imposed on the liquid surface is strong enough to overcome the surface tension of the liquid, the liquid will break up into smaller particles in an attempt to disperse the charge and return to a lower energy state.
Electrospray apparatus are useful for producing very fine nebulized particles of an analyte. The liquid subjected to electrospray techniques might be, for example, a liquid stream effluent from a liquid chromatography (HPLC) separation step. This effluent is passed through an electrospray needle and subject to a strong electric field, which forms a very fine electrospray. The electrospray in this example could be subsequently analyzed by mass spectrometry (MS), which can advantageously provide molecular weight and structural information about the separated species as they emerge from the liquid chromatograph. MS is commonly used to determine molecular weight, identify chemical structures, and accurately determine the composition of mixtures. MS is becoming increasingly important in biological research to determine the structure of organic molecules based on the ion fragmentation pattern formed when sample molecules are ionized by electrospray techniques.
As an added advantage, the electrospray needle can be configured with a lumen that contains a packing material for adsorbing selected chemicals in the liquid solution before the electrospray is discharged from the spray needle. Further known configurations can include pneumatic, thermal, or ultrasonic assist, or the addition of arc suppression gases so that higher voltages can be applied during electrospray formation.
The prior art teaches several electrospray apparatus. Typically, these apparatus comprise a needle which is essentially a very fine capillary that can be as thin as 1-20 .mu.m. The analyte is fed through the capillary and thereafter exposed to an electric field as discussed hereinabove. Needles of similar construction are also utilized in an ion spray process, which is an electrospray process in which the liquid is nebulized by a turbulent flow of gas such as nitrogen. Typically, the field strength required to produce an electrospray requires a voltage bias of about 2.0 to 2.5 kilovolts (kv), usually applied directly to the needle, or to electrodes placed on either side of the needle's orifice.
One serious drawback in prior art electrospray processes is the fragility of the electrospray needle capillary. Because the outlet of the needle is typically around 1-2 .mu.m, the capillary is extremely sensitive to physical disruption and is subject to very easy breakage. This is particularly problematic due to the expense of the capillary. Another serious drawback resides in the fact that the opening of the capillary must be proximate to the electrodes for creating the electric field necessary for electrospray formation. The prior art devices in many instances fix the anode of the field circuit on the needle body itself, which requires the formation of the capillary or its encasement in a conductive material, for example stainless steel. Such constructions are typically expensive. The expensive nature of the construction of the electrospray needles in combination with their fragility makes replacement costs a recurring expense for users of electrospray technologies.
As illustrated in FIG. 1, a typical prior art electrospray needle apparatus 10 is depicted. The capillary needle 12 is configured with a tip 14. The needle apparatus 10 includes a plenum 16 for an incoming liquid sample, an upstream inlet 18 and a downstream liquid outlet 20 in the tip 14. The plenum 16 may be electrically conductive so that a voltage applied to the plenum will allow for the transfer of charge into the liquid stream. Alternatively another upstream electrode can be provided, or charge can be imposed on the capillary needle 12. A voltage is applied and the electrical field thereby produced is arranged to be at its highest at downstream outlet 20. The charge is generally conducted from the plenum 16 or capillary 12, to the liquid sample at the downstream outlet 20 in the tip 14 such that the charge and field at the outlet are high enough to cause the exiting liquid sample to break up into charged droplets to form the electrospray.
As illustrated, the tip 14 is formed having an outlet with a very small diameter. For nano electrospray, a 1-2 .mu.m spraying orifice is required and flow rates of approximately 20 nL/min are not uncommon. Needle tips used for nano electrospray, tend to be even less durable and less likely to withstand handling and the rigors of use in a laboratory setting, without breaking.